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Egg recognition and rejection are the most common and effective anti‐parasitic strategies against avian brood parasitism in terms of maintaining stability over time and plasticity in response to environmental cues. Conversely, parasites have evolved multiple counter‐adaptations to the anti‐parasitic defenses of hosts. Among them, the crypsis hypothesis suggests that eggs that appear more cryptic in color and are closely matched to the environment are helping to counter the egg recognition strategy of the host. In this study, we investigated whether the egg recognition ability of Oriental reed warblers (Acrocephalus orientalis), a common host of common cuckoos (Cuculus canorus), changed during different reproductive stages by using model egg experiments. The effect of the crypsis hypothesis on the egg recognition ability of the hosts was also investigated by controlling the color contrast between the inside of the experimental nests and the model eggs. The results showed that the Oriental reed warbler retained strong egg recognition abilities, which were similar to the incubation stage (GLMMs: F1,27 = 0.424, p = .521), even after entering the nestling stage and preferentially rejected model eggs with distinct contrasting colors (binomial test: Fisher's exact, p = .016). These results are consistent with the crypsis hypothesis. The present study suggests that the host retains a strong egg recognition ability even during the nestling stage and that cryptic‐colored eggs that are closely matched with the breeding nest environment help counter the host's egg recognition abilities and increase the chances of successful parasitism by cuckoos. However, the effectiveness of cryptic egg may be weaker than mimic egg in countering egg recognition and rejection by hosts with open‐cup nests. Oriental reed warblers preferentially rejected model eggs with distinct contrasting colors supporting the crypsis hypothesis.

Obligate avian brood parasites lay their eggs in other bird species' nests, leaving these hosts to rear the parasitic young. To eliminate or reduce the costs of parasitism, many hosts have evolved the ability to recognize and reject parasitic eggs, and most use eggshell color for recognition; however, color discrimination should be more challenging in the low‐light conditions facing cavity‐nesting hosts. Therefore, we hypothesized that instead of color, cavity‐nesting birds could rely on perceived differences in brightness (i.e. luminance) and on light levels in their nests for egg recognition. Specifically, we expected that rejection rates would be higher when foreign eggs are lighter or darker than the hosts' own eggs or when nest light levels are higher. To investigate this hypothesis, we experimentally altered the luminance of 413 similarly colored (blue) experimental model eggs and added these experimental eggs to the nests of cavity‐nesting eastern bluebirds Sialia sialis that typically lay immaculate blue eggs. Contrary to our expectations, neither the perceived luminance of the eggs nor the amount of light in the nest cavity predicted host responses. Therefore, it is unlikely that bluebirds base their antiparasitic decisions on perceived differences in eggshell luminance. Our work provides a foundation for future studies that can inform us about how cavity‐nesting birds perceive salient visual stimuli in the dim light conditions of their nests.

Brood parasitism can be costly to host fitness, which in turn may favour host strategies that decrease these costs. Duck (Anatinae) nests are often parasitized by eggs of other ducks, and one way that hosts can respond to potentially costly brood parasitism is to move parasitic eggs to the clutch periphery, where egg incubation temperatures can be suboptimal relative to the clutch centre. We explored whether red‐breasted mergansers Mergus serrator use discriminatory egg incubation against parasitic eggs laid by conspecifics in a population where conspecific brood parasitism (CBP) is common. We used isoelectric focusing electrophoresis of egg albumen from entire clutches of 12 parasitized nests to identify parasitic eggs. A randomization test pooling identified parasitic eggs (n = 50) across nests revealed that hosts did not position parasitic eggs along the periphery of clutches or out of the central region more than was expected by chance, and this was the case for parasitic eggs laid both before and after the onset of incubation. Similarly, nest‐level analyses showed that parasitic eggs were random in all but the smallest clutch, which contained one identified parasitic egg. Thus, parasitic eggs were not moved to the periphery of heavily parasitized clutches, where egg temperature gradients between central and peripheral regions of nests are expected to be greatest. Only four eggs (< 0.5% of 1276 eggs) were found buried within nest bowls. Eggs that were removed from nests consisted of parasite and host eggs and were more likely along the periphery of clutches prior to their removal than was expected by chance. Our results indicate that discriminatory egg incubation of parasitic eggs is not a well‐developed tactic for defending against CBP in red‐breasted mergansers, though hosts may rely on certain cues to decide which eggs are to be removed from nests (e.g. addled eggs).

Egg rejection often involves a cognitive process of recognizing foreign eggs, which can vary not only between species or among different individuals of the same species, but also within the same individual during different breeding stages, leading to markedly different responses to parasitic eggs. We conducted a comparative study in Wuhan, Hubei, and Fusong, Jilin, China, on the recognition and rejection behavior of azure-winged magpies ( Cyanopica cyanus ) at different breeding stages (pre-egg-laying, one-host-egg, multi-host-egg and early incubation stages). In the Fusong population, there was a significant difference in the rejection rate of model eggs by azure-winged magpies at different stages of the egg-laying period. During the one-host-egg stage, the rejection rate (63.6%) was significantly lower than that during the pre-egg-laying stage (85.7%) and the multi-host-egg stage (100%). The population of azure-winged magpies in Wuhan exhibited a 100% rejection rate towards model eggs during the pre-egg-laying stage. Furthermore, during the incubation stage, azure-winged magpies were able to accurately recognize and reject foreign eggs even when those were in majority. This indicates that azure-winged magpies employ a template-based recognition mechanism rather than relying on discordance mechanism for recognition after the onset of incubation. This study suggests that while azure-winged magpies can truly recognize their own eggs, different breeding stages still influence their rejection response towards parasitic eggs, especially during the pre-egg-laying and egg laying stages.

When the strength or nature of a host-parasite interaction changes over the host life cycle, the consequences of parasitism can depend on host population age structure. Avian brood parasites reduce hosts’ breeding success, and host age may play a role in this interaction if younger hosts are more likely parasitized and/or less able to defend themselves. We analyzed whether the age of female magpie (Pica pica) hosts is associated with parasite attack or their ability to reject foreign eggs. We recorded parasitism and model egg rejection of known-age individuals over their lifetime and established whether the likelihood of parasitism or egg rejection changed with age or longevity. Parasitism probability did not change with female age, and there was a trend toward longer-lived females being less likely to be parasitized. However, model egg rejection probability increased with age for each individual female, and longer-lived females were more prone to reject model eggs. Most females in the population were young, and the majority of them accepted model eggs, suggesting that brood parasites exploiting younger host individuals are benefitting from a lower defense level of their hosts. Our results stress that the intensity of selection by brood parasites may be mediated by the age structure of host populations, a to-date neglected aspect in brood parasite–host research.

Obligate avian brood parasitism is a reproductive strategy in which parasitic birds rely entirely on host species for incubation and chick rearing, reducing host reproductive success. This pressure has led many host species to evolve defenses, particularly the ability to recognize and reject parasitic eggs. However, not all individuals within a host population exhibit this behavior—some accept parasitic eggs, while others reject them. One proposed explanation for this behavioral variation is genetic polymorphism. Previous research on Eurasian magpies ( Pica pica ) linked egg rejection behavior to a specific microsatellite locus (Ase64), though these studies used genotypes from nestlings, not adult females, who are the ones expressing the behavior. Since egg rejection can change with age, accurately categorizing females as acceptors or rejecters requires repeated observations across their lifespan. This study genotyped adult females whose behavior had been reliably classified over their lifes to test for genetic differences. Results showed significant differences in the genotypic and allelic frequencies at the microsatellite marker MSLp4, which was more common in acceptors. These findings support the hypothesis that a genetic polymorphism contributes to individual variation in egg rejection behavior, a defensive trait that may shift over an individual’s lifetime in magpies.

Egg retrieval behavior in hosts within avian brood parasitism systems was found to be regulated by the motivation to reject parasitic eggs. However, due to the limitations in the research systems, there is a lack of effective validation regarding the adaptation mechanisms of cuckoo hosts to the conflict between retrieving their own eggs outside the nest and rejecting parasitic eggs. This study uses Daurian redstarts (Phoenicurus auroreus), a secondary cavity‐nesting host parasitized by common cuckoos (Cuculus canorus), to verify the adaptive decision‐making of the host between egg retrieval and egg rejection by simulating the occurrence of eggs outside the nest. The results showed that Daurian redstarts ignored 60.6% of highly mimetic conspecific eggs, with a retrieval rate of only 18.2%. Additionally, Daurian redstarts rejected 21.2% of conspecific eggs. However, non‐mimetic budgerigar (Melopsittacus undulatus) and white model eggs were more likely to be directly rejected (75% and 86.4%, respectively) with no retrieval events. Our findings suggest that egg retrieval behavior in Daurian redstarts is likely influenced by the cognitive process of rejecting parasitic eggs, leading to occasional over‐identification and difficulty in decision‐making between egg retrieval and egg rejection, especially in the context of conflicting motivations. Little work on the cuckoo hosts adapting mechanism to the conflict between ignoring own eggs and rejecting parasitic eggs. The Daurian redstart (Phoenicurus auroreus), a cuckoo host, ignored most conspecific eggs with only a retrieval rate of 18.2%, while it rejected most non‐mimetic eggs with no egg retrieval. This is the first study showing that the behavior of ignoring eggs outside the nest in cuckoo hosts is related to egg recognition abilities and egg retrieval could be a cognitive indicator.

Coevolutionary arms races are a potent force in evolution, and brood parasite-host dynamics provide classical examples. Different host-races of the common cuckoo, Cuculus canorus, lay eggs in the nests of other species, leaving all parental care to hosts. Cuckoo eggs often (but not always) appear to match remarkably the color and pattern of host eggs, thus reducing detection by hosts. However, most studies of egg mimicry focus on human assessments or reflectance spectra, which fail to account for avian vision. Here, we use discrimination and tetrachromatic color space modeling of bird vision to quantify egg background and spot color mimicry in the common cuckoo and 11 of its principal hosts, and we relate this to egg rejection by different hosts. Egg background color and luminance are strongly mimicked by most cuckoo host-races, and mimicry is better when hosts show strong rejection. We introduce a novel measure of color mimicry—\" color overlap\"—and show that cuckoo and host background colors increasingly overlap in avian color space as hosts exhibit stronger rejection. Finally, cuckoos with better background color mimicry also have better pattern mimicry. Our findings reveal new information about egg mimicry that would be impossible to derive by the human eye.

In species that are subject to brood parasitism, individuals often vary in their responses to parasitic eggs, with some rejecting the eggs while others do not. While some factors, such as host age (breeding experience), the degree of egg matching and the level of perceived risk of brood parasitism have been shown to influence host decisions, much of the variation remains unexplained. The host personality hypothesis suggests that personality traits of the host influence its response to parasitic eggs, but few studies have tested this. We investigated the relationship between two personality traits (exploration and neophobia) and a physiological trait (breathing rate) of the host, and egg-rejection behaviour in a population of Daurian redstarts Phoenicurus auroreus in northeast China. We first show that exploratory behaviour and the response to a novel object are repeatable for individual females and strongly covary, indicating distinct personality types. We then show that fast-exploring and less neophobic hosts were more likely to reject parasitic eggs than slow-exploring and more neophobic hosts. Variation in breathing rate—a measure of the stress-response—did not affect rejection behaviour. Our results demonstrate that host personality, along the bold-shy continuum, predicts the responses to parasitic eggs in Daurian redstarts, with bold hosts being more likely to reject parasitic eggs.

Brood parasitism negatively affects the reproductive success of hosts, leading many hosts to evolve defense strategies to recognize and reject parasitized eggs. While studies have shown that hosts may adjust their defensive behavior according to parasitism risk, whether different geographical populations of the azure‐winged magpie (Cyanopica cyanus), which are parasitized by multiple cuckoo species and face varying parasitism risks, exhibit geographical variation in egg rejection behavior is unclear. As studies have shown that the color of model eggs can influence the egg rejection behavior of hosts, we also tested whether red or blue model eggs would affect the egg rejection behavior of the azure‐winged magpies. From April to June in 2023, we investigated the egg recognition ability of azure‐winged magpie populations in Fusong County, Jilin Province; Huangpi District, Hubei Province; and Luqu County, Gansu Province, China. The results showed that the azure‐winged magpie populations in all three regions rejected approximately 100% of the model eggs, with no significant difference in rejection rates between red and blue model eggs. This study revealed that Chinese populations of azure‐winged magpies exhibited high egg recognition ability. There was no geographical variation in their egg rejection behavior when presented with non‐mimetic blue or red model eggs. This study provided basic data for further research on the anti‐parasitic strategies of the azure‐winged magpie. Hosts may adjust their defensive behavior according to parasitism risk. Whether the azure‐winged magpie (Cyanopica cyanus) exhibits geographical variation in egg rejection behavior is unclear. Regardless of cuckoo parasitism status, all three azure‐winged magpie populations across China exhibited high recognition ability for non‐mimetic eggs, with no geographical variation in rejection behavior.